Electrostatic printing



' liquid developer.

Stes

3,041,163 ELECTROSTATIQ PRINTING Henry Wieliclri, Philadelphia, Pa, assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Filed Sept. 18, 1959, Ser. No. 840,801

, 2 Claims. (Cl. 96-1) This invention relates generally to electrostatic printing. More particularly, it relates to improved recording elements, and methods of preparing such recording elements for use in electrostatic printing.

In the art of electrostatic printing, electrostatic images are produced on the surface of a photoconducting insulating material. This frequently is accomplished by first producing on the surface a substantially uniform electrostatic charge, as for example, by exposure to -a corona discharge device. Upon exposure to a light image the electrostatic image is produced comprising areas bearing electrostatic charges and 'other areas wherein electrostatic charges have been substantially reduced or removed by the exposureim light. Visible images are produced same charge image surface by the electrostatic attraction of finely-divided developer particles to that surface. Methods of developing electrostatic images are described in Electrofax Direct Electrophotogr'aphic Printing On Paper, by C. J. Young and H. G. Greig, 'RCA Review, December 1954, vol. XV, No. 4.

' So called liquid processes for developing electrostatic images have been proposed inwhich the solid developer particles are suspended in aninsulating carrier liquid. A liquid development process is described by K. A. Metcalf and R; J. Wright in a paper entitled Xerography, published inthe Journal of the Oil and Colour Chemists Association, November 1956, vol. 39, No. 11, London,

England and in another paper entitled Liquid Developers for Xerography published in the Journal of Scientific Instruments, February 1955, vol. 32. Briefly, the liquid developer described by Metcalf an Wright consists of finely-divided developer particles dispersed in' an insulating liquid. This liquid developer can be flowed over a surface bearing an electrostatic image, or the surface can be immersed in a tray of The liquid developer may also be sprayed or rolled on to the surface. .Deposition of the developer particles on the charge image is an example of the phenomenon known as electrophoresis or cataphores is. v

One type of recording element usable in the foregoing electrostatic printing processes comprises a backing member coated with an electrically-insulating photoconductive layer .such as a particulate photoconductive zinc oxide suspended in a resinous polysiloxane (silicone) binder. A coating also has been developed which comprises a particulate photoconductive zinc oxide suspended in a'binder of a film-forming resinous styrene-butadiene copolymer or in a binder mixture comprising a major proportion of the copolymer and a minor proportion of another insulating resinous material or wax which is compatible with the copolymer. An important advantage of coatings containing a styrene-butadiene copolymer expensive.

is one of cost, resins such-as silicone being much more 'Although desirable from the standpoint of cost, it has been found that photoconductive coatings containing a styrene'butadiene copolymer are not suitable for all electrostatic printing applications. I 'When charged with a corona discharge device ind arkness (or with a safe light) using copolymer coatings are sometimes unsatisfactoryfi For example, in electrostatic printing processes using liquid developer dispersions such as those described in Metcalf and Wright, op. cit., uneven and inadequate deposit of developer particles on the charged areas of the coating results. The finished print in such a case is deficient in intensity and presents a blotchy appearance, particularly when large areas are developed.

Accordingly, it is a general object'of this invention to provide improved electrostatic printing on a surface of a photoconductive insulating'layer which includes a filmforming styrenebutadiene copolymer as a major constituent thereof.

It is another object of this invention to provide improved photoconductive recording elementsand coatings containing a resinous film-forming styrene-butadiene copolymen.

It is a further object of this invention to provide improved methods of preparing electrophotographic re cording elements comprising a :backing member coated with a finely-divided pho-toconductor dispersed in a resinous binder at least a major portion of which is a film-forming styrene-butadiene-copolymer.

It is yet another object of this invention to provide improved methods of electrostatic pr-inting on the surface of a photoconductive layer which includes a resinous film-forming styrene-butadiene copolymer.

The foregoing objects and other advantages are accomplished in accordance with this invention which is based on the ,discovery that pretreatment of photoconductive coating compositions containing a resinous filmforming styrene-butadiene copolymer with a selective solvent provides unusual and unexpected results. The class of coatings or layers which have been found to be improved by such treatment includes those comprising a finely-divided photoconductoi dispersed in a resinous binder of a film-forming styrene-butadiene copolymer or wherein the resinous binder comprises a mixture of a major portion of fihn-forming styrene-butadiene copoly of the photoconductive layer may be accomplished by immersion,'spraying or washing techniques.

Specific examples and additional advantagesare in} cluded in the detailed description which follows:

Photoconductive coatings which can be improved in accordance with this invention comprise'lOO'to 900 parts,

by weight, of a finely-divided photoconductor dispersed in about parts, by weight, of binder. The binder cornprises a resinous film-forming styrene-butadiene copolymer including 60 to 92%, by weight, styrene and 40 to" 8%, by Weight, butadienel Such a coating may be pre pared according to the process disclosed in U.S. Patent 2,611,719, issued to Alvin'M. Borders, or the process described in U.S. Patent2,537,1l'4, issued to A .E.Y oi.1ng,

et al.

Example 1 A coating mixture is prepared of the following formu;

lation in part by weight.

45 parts photoconductive zinc-oxide (FlorenceGreen Seal 8, marketed by the New Jersey Zinc Sales Co, New;

York, New York). I p

15 parts, by weight, Pliolite S-SD, a high styrenebutadiene copolymerv marketed by the Goodyear Tire and Rubber Co., Akron, Ohio.

1.5 parts plasticizer. 100 parts toluene.

Patented June 26, 1962 and is coated on the surface of a backing member such as, for example, paper. Any conventional coating process may be used to provide a final coating thickness of from about 0.0003 to about 0.002 inches. The coating is air dried or heat may be used to accelerate the drying step. Once dried, this coating has applied to it, a volatile solvent such as, for example, trichlorotrifluoroethane. Upon evaporation of the volatile solvent from the surface of the coating, the recording element is ready for use in electrostatic printing.

Any solvent having a kauni-butanol number of from about 20 to about 40 may be applied to the coating to provide improved results. In adition to trichlorotrifiuoro'ethane (kauri-butanol Number 31), specific examples include the following:

Kauri-butanol No.

Petroleum ether 25 Pentane 25 Isohexane 27.5 Hexane' 30 Iso-octane 32 Octane V35 Isoheptane 35.5 Heptane 38 It has been found that solvent-treatment as described above produces a substantially permanent improvement in the electrical properties of the photoconductive coating. Hence, solvent-treatment may be incorporated in a process for producing improved recording elements or, in the alternative, solvent-treatment may be incorporated in an electrostatic printing process. It has also been found that duration of treatment is not critical. For example, a recording element prepared as described in Example I may be given a quick dip in a quantity of the solvent or it may be immersed therein for several hours without any noticeable difference in results.

The solvent-treated recording element may be used for electrostatic printing by any of the methods described in the Young and Greig publications or in the Metcalf and Wright publications op. cit. By Way of example, the recording element may be placed with its pap er backing on a grounded metal plate and in darkness an electrostatic charging device passed over the photoconductive coating to provide a substantially uniform electrostatic charge thereon. The next step in the process is to discharge selected areas of the charge surface of the recording elenrent in order to produce an electrostatic image thereon. This may be accomplished by placing a photographic transparency upon the charged surface and exposing it to light through the transparency. Other exposure processes also may be used such as, for example, projection and reflection exposure. Wherever light strikes the surface of the photooonductive coating, the electrostatic charge thereon is substantially reduced or removed to create a pattern of charges corresponding to the nonilluminated areas.

The electrostatic charge pattern may be converted into a visible image by any of the dry powder techniques described in the Young and Greig publication, or by any of the so-called liquid development techniques described in the Metcalf and Wright publications op. cit. It is during development of the electrostatic image that the improved performance of the photoconducting coating de-' scribed herein becomes evident. Without treatment of the photoconducting surface with a volatile solvent, fewer particles of development material are attracted to the coating which has been pretreated, unevenness of deposition is not noticeable and the density of the image is substantially increased.

Additional examples of photoconductive coatings which may be prepared in the same manner as Example I and which may be improved by the application thereto of one of the forernentioned solvents include the follow- Example 2 1 Parts Florence Green Seal-8 zinc oxide, marketed by the New Jersey Zinc Sales Co., New York, NY. 45 Piccopale 100 resin, a methylated paraflin resin marketed by the Pennsylvania Industrial Chemical Co., Clairton, Pa. 6

Pliolite S-SD, marketed by the Goodyear Tire and Rubber Co., Akron, Ohio 9 F-2l Plasticizer, marketed by the National Lead Co.,

New York, NY. 1.5 Toluene 100 Example 3 Parts Florence Green Seal-8 zinc oxide 125 Pliolite S-S 30 Piccopale 100 resin l0 Aroclor 1254, plasticizer marketed by Monsanto Chemical Co., Organic Chemicals Division, St. Louis 7, Missouri 1.0 Toluene 100 Example 4 Parts Florence Green Seal-8 zinc oxide 125 Pliolite S-S 25 Piccopale 100 resin 5 Aroclor 5460 (a chlorinated polyphenyl resin soluble in toluene) marketed by Monsanto Chemical Co. 20 Toluene 120 40 Example 5 Parts Florence Green Seal-8 zinc oxide 125 Pliolite S5 1 30 Aroclor 5460 20 Toluene 140 Example 7 7 Parts Florence Green Seal-8 zinc oxide 60 Pliolite s-s resin 9 Picoopale 100 resin 6 F-Zl Plasticizer 0.5 Toluene 100 Cellosolve acetate 5 RoseBengal (Cl. 779) dissolved in methyl ethyl ketone 0.24

Example 7 Parts Florence Green Seal-8 zinc oxide 150 Pliolite S-S resin 25 Cumar T-3 a synthetic coumarone indene resin marketed by Barrett Division, Allied Chemical and Dye Corporation, 40 Rector Street, New York '6, NY. 16 Tricresyl phosphate plasticizer 9 Toluene 200 Example 8 0 Parts Florence Green Seal-8 oxide 45 Pliolite S5D 9 Piccopale 6 F-2l plasticizer 0.75 75 Toluene It is not known Why coatings containing styrene-butadiene copolymers are improved as described herein other than that the charge storage capability of such coatings is greatly enhanced. In actual tests, one sample was found to be capable of being charged to a voltage of 360 volts in darkness, whereas before it was treated with a solvent it could only be charged up to 160 volts. Another sample evidenced improvement of from 200 up to 400 volts as a result of solvent treatment. It is known, however, that pretreatment of a resin prior to the formulation of a coating composition does not produce the same result. Such a test was made by immersing a powder of Pliolite S-SD in trichlorotrifluoroethane for a time sufiieient for the solvent to thoroughly wet all of the Pliolite resin. The resin was then dried, dissolved in toluene and a photooonductive zinc oxide dispersed therein. This mixture was coated on paper following which prints were made thereon with liquid developer dispersions. It was found that such pretreatment of the resin provided no enhancement of development. It was further fiound that subsequent immersion of such a coated paper in trichlorotrifluoroethane or the other solvents described herein did enhance development results. No explanation can be given for this phenomenon other than that the solvent treatment of a coated recording element must result in some physical reordering of the photo conductive surface which may result in an effect equivalent to providing-a substantially greater surface area capable of retaining an electrostatic charge.

What is claimed is:

1. In a method of electrostatic printing including the steps of producing a latent electrostatic image on the surface of a photoconductive insulating material comprising a finely-divided photoconductor suspended in a filmforming binder at least a major portion of which consists of a resinous styrene-butadiene copolymer and applying to said surface a finely-divided developer substance to produce thereon a visible image; the improvement comprising applying to said surface a solvent the active element of which is selected from the class consisting of trichlorotrifluoroethane, petroleum ether,

pentane, isohexane, hexane, isoheptane, iso-octane, octane,

and mixtures thereof, and drying said coating prior to said step of producing said latent electrostatic image.

2. The method of claim 1 wherein said solvent is trichlorotrifluoroethane.

References Cited in the file of this patent UNITED STATES PATENTS 2,692,948 Lion Oct. 26, 1954 2,875,054 Griggs et :al. Feb. 24, 1959 2,891,911 Mayer et al June 23, 1959 2,892,709 Myer June 30, 1959 2,904,431 Moncreifi-Yeates Sept. 15, 1959 OTHER REFERENCES Payne: Organic Coating Technology, vol. 1, page 258.

Wiley and Sons, Inc., New York, 1954. 

1. IN A METHOD OF ELECTROSTATIC PRINTING INCLUDING THE STEPS OF PRODUCING A LATENT ELECTROSTATIC IMAGE ON THE SURFACE OF A PHOTOCONDUCTIVE INSULATING MATERIAL COMPRISING A FINELY-DIVIDED PHOTOCONDUCTOR SUSPENDED IN A FILMFORMING BINDER AT LEAST A MAJOR PORTION OF WHICH CONSISTS OF A RESINOUS STYRENE-BUTADIENE COPOLYMER AND APPLYING TO SAID SURFACE A FINELY-DIVIDED DEVELOPER SUBSTANCE TO PRODUCE THEREON A VISIBLE IMAGE; THE IMPROVEMENT COMPRISING APPLYING TO SAID SURFACE A SOLVENT THE ACTIVE ELEMENT OF WHICH IS SELECTED FROM THE CLASS CONSISTING OF TRICHLOROTRIFLUOROETHANE, PETROLEUM ETHER, PENTANE, ISOHEXANE, HEXANE, ISOHEPTANE, ISO-OCTANE, OCTANE, AND MIXTURES THEREOF, AND DRYING SAID COATING PRIOR TO SAID STEP OF PRODUCING SAID LATENT ELECTROSTATIC IMAGE. 